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Polyribosomes (or polysomes) also known as ergosomes are a cluster of ribosomes, bound to a mRNA molecule, first discovered and characterized by Jonathan Warner, Paul Knopf, and Alex Rich in 1963. Many ribosomes read one mRNA simultaneously, progressing along the mRNA to synthesize the same protein. They may appear as clusters, linear polysomes, or circular rosettes on microscopy, but mainly circular in vivo. This circularization is aided by the fact that mRNA is able to be twisted into a circular formation, creating a cycle of rapid ribosome recycling, and utilization of ribosomes. The 5' 7-methylguanosine cap and 3' poly(A) tail present on eukaryotic mRNA aid in this process.

Polyribosomes can be found in three forms: free, cytoskeletal bound, and membrane bound.